On Tue, Jun 9, 2015 at 3:10 AM, Michael Haggerty <mhag...@alum.mit.edu> wrote:
> On 06/08/2015 06:43 PM, Stefan Beller wrote:
>> On Mon, Jun 8, 2015 at 4:45 AM, Michael Haggerty <mhag...@alum.mit.edu>
>> wrote:
>>> [...]
>>> +/*
>>> + * Delete the specified reference. If old_sha1 is non-NULL and not
>>> + * NULL_SHA1, then verify that the current value of the reference is
>>> + * old_sha1 before deleting it.
>>
>> And here I wondered what the distinction between NULL and non-NULL,
>> but NULL_SHA1
>> is and digging into the code, there is none. (As you can also see in
>> this patch above with
>> (old_sha1 && !is_null_sha1(old_sha1)) ? old_sha1 : NULL,
>> but when digging deeper, the !is_null_sha1(old_sha1) is an arbitrary
>> limitation (i.e.
>> ref_transaction_delete and ref_transaction_update don't differ between
>> those two cases.)
>>
>> The distinction comes in at lock_ref_sha1_basic, where I think we may
>> want to get rid of
>> the is_null_sha1 check and depend on NULL/non-NULL as the difference
>> for valid and invalid
>> sha1's?
>
> I'm having a little trouble understanding your comment.
>
> The convention for ref_transaction_update() and friends is that
>
> * old_sha1 == NULL
>
> We don't care whether the reference existed prior to the
> update, nor what its value was.
>
> * *old_sha1 is NULL_SHA1
>
> (by which I mean that old_sha1 points at 20 zero bytes; I hope
> that's clear even though NULL_SHA1 is not actually defined
> anywhere): The reference must *not* have existed prior to the
> update.
Ok that's what I was missing.
>
> * old_sha1 has some other value
>
> The reference must have had that value prior to the update.
>
> lock_ref_sha1_basic() distinguishes between { NULL vs. NULL_SHA1 vs.
> other values } in the same way that ref_transaction_update() does.
>
> The delete_ref() function doesn't follow the same convention. It treats
> NULL and NULL_SHA1 identically, as "don't care".
>
> It probably makes sense to change delete_ref() use the same convention
> as ref_transaction_update(), but there are quite a few callers and I
> didn't have the energy to review them all as part of this patch series.
> So I left it unchanged and just documented the status quo better.
>
>> That said, do we want to add another sentence to the doc here saying
>> non-NULL and not
>> NULL_SHA1 are treated the same or is it clear enough?
>> With or without this question addressed:
>> Reviewed-by: Stefan Beller <sbel...@google.com>
>
> In set notation,
>
> "non-NULL" =
> "non-NULL and not NULL_SHA1" ∪
> "non-NULL and equal to NULL_SHA1"
>
> The latter two are *not* treated the same, so I don't see how we can
> claim that "non-NULL" and "not NULL_SHA1" are treated the same. I must
> be misunderstanding you.
>
> Would it help if I changed the comment to
>
> Delete the specified reference. If old_sha1 is non-NULL and not
> NULL_SHA1, then verify that the current value of the reference is
> old_sha1 before deleting it. If old_sha1 is NULL or NULL_SHA1,
> delete the reference it it exists, regardless of its old value.
>
> ?
This is very clear to me.
>
> Michael
>
> --
> Michael Haggerty
> mhag...@alum.mit.edu
>
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